Database
[{"data":{"name":"Doe","values":{"AA_Avg":13,"BB_Avg":19,"CC_Avg":18}}}]
Query
db.collection.update({},[{$set:{"data.values":{$arrayToObject:{$map:{input:{$objectToArray:"$data.values"},in:{k:{$replaceOne:{input:"$$this.k",find:"Avg",replacement:"MT"}},v:"$$this.v"}}}}}}])